Alternate fuels have been developed to mitigate the rising prices of conventional fuels, to reduce dependence on imported fuels, and for reducing production of pollutants, such as CO2. For example, alcohol and alcohol-based fuel blends have been recognized as attractive alternative fuels, in particular for automotive applications. However, alcohol, alcohol-based fuels, and gasoline are less volatile than diesel, and as such may not evaporate effectively during engine cranking at cold-start conditions. A higher amount of fuel may be supplied during cold-start to provide a desired air fuel ratio for combustion. Incomplete vaporization of the alcohol and alcohol-based fuels may reduce fuel economy and degrade emissions.
Various approaches are provided for increasing fuel vaporization during cold-start. In one example, as shown by Samejima in JP 2009002314A, an engine cranking speed is adjusted. In particular, a higher engine cranking speed is applied when the alcohol concentration of the injected fuel is high and the ambient temperature is low. In another example approach shown by Kuroki in JP 2008232007, a starter motor speed is increased when there is an issue with fuel vaporization. In a further example, Ulrey et al. in U.S. Pat. No. 9,346,451 disclose a method of cranking the engine unfueled at a lower than normal speed such that the heat generated in the compression stroke of a cylinder may be transferred to cylinder walls, thereby expediting engine warm-up.
However, the inventors herein have recognized potential issues with such approaches. In the approaches shown by Samejima and Kuroki, the starter speed is increased to rapidly reduce the intake manifold pressure since the lower pressure assists in fuel vaporization. However, the rapid reduction in manifold pressure via the increasing of the starter speed also reduces the time available for vaporizing the fuel. Consequently, it may be difficult to optimize the starter speed for both the manifold pressure and the fuel alcohol content. Also, increased cranking speed during engine start may result in engine flares. An optimal amount of vaporized fuel may be desired to maintain combustion stability. Incomplete fuel vaporization may further lead to cylinder misfiring events. In the approach shown by Ulrey et al., since the engine is cranked unfueled at a lower cranking speed, once fueling is initiated, fuel blends with a higher alcohol content may not get sufficient time for vaporization before combustion is initiated. If a higher amount of fuel is injected to ensure availability of a desired amount of vaporized fuel during combustion, a portion of the un-vaporized fuel may form wall films in the combustion chamber. Such un-vaporized fuel may be released to the atmosphere along with exhaust gas, thereby increasing unburnt hydrocarbon (UHC) and particulate matter (PM) emissions. During cold-start conditions, the exhaust catalysts may not be optimally functional and therefore may not be efficient in reducing UHC and NOx emissions. Further, an increased amount of fueling may adversely affect fuel efficiency.
In one example, the issues described above may be addressed by an engine method comprising: during cold-start, for a lower than threshold fuel boiling point, cranking the engine via a starter motor with a first cranking speed while injecting fuel for a number of engine cycles since a first engine cycle, and for a higher than threshold fuel boiling point, cranking the engine with a second cranking speed while injecting fuel and disabling spark for the number of engine cycles since the first engine cycle. In this way, by cranking the engine via a starter motor at a lower cranking speed without increased amount of fueling, sufficient time may be provided to vaporize the fuel and provide a homogeneous air-fuel mixture.
As one example, during cold-start conditions, a starter motor may be actuated to crank the engine. The cranking speed may be lowered relative to a nominal cranking speed while fuel is injected into the engine. The lowering of the cranking speed may be adjusted based on the boiling point of the fuel, the cranking speed decreased with an increase in the fuel boiling point. For fuels with a higher than threshold boiling point, in addition to lowering cranking speed and injecting fuel, spark may be disabled for a number of engine cycles The number of engine cycles and the cranking speed may be selected based on the boiling point of the injected fuel and the ambient temperature so as to enable a larger portion of the fuel to be vaporized by the time spark is enabled. As an example, the cranking speed may be lowered to 150 rpm, and the engine may be fueled with no spark for a number of engine cycles (e.g., the first two engine cycles since engine start is initiated). On the subsequent engine cycle (e.g., the third engine cycle since the engine is started), the cranking speed may be raised, for example to 250 rpm, and spark may be resumed. Also, for fuels with a higher than threshold boiling point, to further improve fuel vaporization, fuel injection timing may be adjusted to extend up till the spark event. For example, an end of fuel injection timing may be shifted from bottom dead center (BDC) of the intake stroke to top dead center (TDC) of compression stroke.
In this way, by lowering cranking speed to below the nominal speed, a larger time window is provided for fuel vaporization. Also, by using a lower engine cranking speed, engine speed flares may be reduced. The technical effect of increasing fuel vaporization by lowering cranking speed to below the nominal speed is that a lower total amount of fuel may be injected to obtain the desired amount of vaporized fuel, thereby reducing an amount of un-vaporized fuel being released to the atmosphere and improving emissions quality. By reducing the amount of fuel injection, fuel efficiency may be improved. For higher boiling point fuels, by deactivating spark until a defined number of fueled engine cranking cycles have elapsed, each cylinder may be conditioned with vaporized fuel and upon activating spark after accumulation of an optimal amount of pre-vaporized fuel, combustion stability may be improved. By improving combustion stability, misfire event occurrence and further unburned hydrocarbon emissions during engine starts may be reduced. By increasing the cranking speed after the number of engine cycles have elapsed, the desired intake manifold pressure may be attained, facilitating combustion. By adjusting the cranking speed, fuel injection profile, and the number of non-firing cycles based on the boiling point of the fuel, vaporization of any variety of gasoline or alcohol based fuel may be optimized. Overall, by increasing the degree of fuel alcohol vaporization, engine performance, fuel economy, and emissions quality may be increased.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.